Taphole plugging compositions (muds) are prepared by kneading a mixture comprising a refractory aggregate such as alumina, silicon carbide, bauxite, mullite, chamotte, pyrophyllite or coke, using a binder such as tar or phenolic resin. The compositions are used to stop, when necessary, tapping of molten metal by closing tapholes through which molten metal is discharged from a metal melting apparatus. After a certain period of time from stoppage of tapping, a passageway for molten metal is formed through the compositions, so that tapping can be easily restarted. Therefore, taphole plugging compositions need to have various properties as described below.
Usually, a taphole plugging composition is packed into a taphole using an extruding tool called a mud gun. With insufficient plasticity, the plugging composition is incapable of being extruded from the mud gun or incapable of sufficiently plugging the taphole, failing to stop tapping. Thus, the plugging composition is required to have a suitable plasticity and to achieve a suitable density after plugging.
Further, the plugging composition is required to have, when being packed into a taphole, properties of being rapidly sintered (sinterability) by the heat in the metal melting apparatus (furnace) and a sufficient sintering strength to prevent molten metal from flowing out. A plugging composition with a poor sinterability does not sufficiently stop tapping, and molten metal will flow out and cause problems. Further, the plugging composition needs to have a high porosity (high gas permeability) after sintering, since the binder is rapidly gasified during solidification of the composition and is liable to make the composition disintegrate or coarsen.
After a certain period of time from the stoppage of tapping, a hole is bored through the sintered plugging composition using a drill, to restart tapping. When the composition has an excessively high sintering strength (strength after solidification), it will be time-consuming to bore a hole through the composition. Therefore, the composition needs to have a suitable sintering strength. Further, low-polluting properties are also important for the plugging composition, since smoke or dust is produced during boring.
Moreover, since molten metal passes through the bored hole, the sintered plugging composition is required to have a sufficient strength and corrosion resistance to withstand contact with molten metal, and a good adhesion strength to bricks forming the wall of the taphole. If these properties are insufficient, the diameter of the bored hole is enlarged, and molten metal is discharged scatteringly, making tapping unstable. In the worst case, the furnace must be closed before completion of tapping.
As binders for the plugging composition, tars and pitches have been long used because they are inexpensive.
"Tar" is a black or dark brown viscous oily bituminous material produced by heat treatment or pyrolysis of organic materials such as coal or wood. Tars include coal tars (oils produced by dry distillation of coal), shale tars (oils produced by dry distillation of oil shale), wood tars, oil gas tars (oils produced as byproducts of petroleum gasification), petroleum tar (a generic name of vaporization or distillation residues of petroleum or its pyrolysates, including petroleum asphalt, still residue, pyrolysis tar and the like). "Pitch" is a black carbonaceous solid residue obtained by heat treatment or pyrolysis (e.g., distillation such as dry distillation) of organic materials.
When a tar or a pitch is used as a binder for the plugging composition, the resulting composition has a good adhesion to bricks forming the taphole wall, but takes much time to sinter and is therefore liable to cause problems such as leakage of molten metal. Further, tars and pitches have a serious problem of heavy smoke and dust produced at the time of plugging or opening of the taphole.
Phenolic resins are also used as binders for the plugging composition. A plugging composition prepared using a phenolic resin has a quick drying characteristic (quick volatility of liquid components) and a high sintering strength. However, because of the excessively high sintering strength, it is difficult to bore a hole through the composition. Moreover, since phenolic resins are expensive, a plugging composition prepared using a phenolic resin is less economical than those prepared using inexpensive tars.
A combined use of a phenolic resin and a pitch has been developed recently, since each of these binders has advantageous properties as described above. However, a plugging composition comprising these binders in combination is insufficient in gas permeability (porosity) and adhesion strength to bricks forming the furnace wall.